[Ir(tpy)(bpy)Cl] as a Photocatalyst for CO2 Reduction under Visible-Light Irradiation

Mononuclear iridium(III) terpyridine (tpy) 2,2-bipyridine (bpy) [Ir(tpy)(bpy)Cl](2+) photocatalysts (denoted [Ir(bpy)]) were developed for selective CO2 reduction to HCOOH under visible light. The CO2 reduction product could be changed dramatically by substituting a 2-phenylpyridine (ppy) ligand with bpy, with the mononuclear Ir ppy complex ([Ir(tpy)(ppy)Cl](+)) acting as a photocatalyst for selective CO2 reduction to CO. A mechanistic study showed a structural change in [Ir(bpy)] during the photocatalytic reaction. The [Ir(bpy)] complex was transformed into an iridium-hydride complex ([Ir(tpy)(bpy)H](2+)) during an early stage of the photocatalytic reaction. However, [Ir(tpy)(bpy)H](2+) did not function as a key intermediate in the photochemical CO2 reduction because an additional structural change occurred. The tpy ligand of the Ir complex was reduced to piperidine-2,6-di-2-pyridine ligand during the photocatalytic reaction, resulting in the production of [Ir(piperidine-2,6-di-2-pyridine)(bpy)H](2+), which was the actual photocatalyst for HCOOH production.